The present invention relates to a video signal transmission apparatus and a video signal transmission method suitable for transmission of a video signal via a wire transmission line such as a coaxial cable.
In a conventional studio camera facility, a FPU (Field Pick Up) apparatus and the like, a modulated video signal is transmitted and received between a studio camera and a camera control unit and between a transmitting/receiving antenna (head unit) and a FPU control unit via a coaxial cable. In the FPU and the like, the coaxial cable installed between the head unit and the control unit is as long as 300 m at its maximum.
In the same way, the studio camera facility or the like is arranged as shown in FIG. 7. That is, a studio camera 1 is installed within a studio 2. A video signal picked up by a camera head 4 which is in turn mounted on a studio adaptor 3 and a picture video signal monitored on a view finder 5 are transmitted/received between a control panel 8 consisting of a switcher and a control console installed in a control room 7, and a camera control unit 9 via a coaxial cable 6.
Such a coaxial cable 6 is pulled about over a considerable length (about 100 m) in the studio 2. Therefore, the video signal transmitted through this coaxial cable 6 is significantly reduced as the frequency thereof becomes higher, resulting in a transmission line characteristic 32 as shown in FIG. 9B. Attenuation amounts of these coaxial cables 6 differ depending upon the characteristic impedance of the coaxial cable 6. However, it is well known that the attenuation amount of the frequency characteristic of a coaxial cable 6 having a characteristic impedance of 75 xcexa9 increases in proportion to {square root over (f)} in the television signal frequency band, if f represents its frequency.
When an analog video signal subjected to, for example, amplitude modulation is to be transmitted to the camera control unit 9 or the like via such a coaxial cable 6, its high frequency component is attenuated in the coaxial cable 6 and consequently the video signal waveform transmitted cannot be reproduced on the side of the camera control unit 9. Conventionally, therefore, the analog video signal was passed through a pre-emphasis circuit having a frequency characteristic raised beforehand in the high frequency component so as to have an inverse characteristic with respect to the transmission characteristic (FIG. 9B) of the coaxial cable 6. Thus the characteristic in the transmission line such as the coaxial cable was canceled.
Such a conventional configuration that the attenuation of the high frequency component in the coaxial cable 6 is compensated by the above described pre-emphasis circuit will now be described by referring to FIG. 8.
In FIG. 8, a picked signal by the camera 1 is digitized, and made as an input signal 30, which is a serial digital signal of 80 Mbps as shown in FIG. 11A, and then input to the camera control unit 9 via an input terminal 10. This input signal 30 is supplied to a modulator 11. In a configuration of, for example, a DDS (Direct Digital Synthesize) system, the modulator 11 conducts amplitude modulation on a carrier having a carrier frequency of 80 MHz by using the serial digital signal supplied as the input signal. As a result, there is obtained a modulated signal having a bandpass characteristic 34 having a bandwidth extending from f1 to f2 with a carrier frequency fc as a center, as shown in FIG. 9A. The modulated signal is transmitted via the coaxial cable 6.
The above described modulator 11 is a digital modulator. Since the input signal is a digital signal, its harmonic components extend to high frequencies. In order to extract the band of the baseband (FIG. 9A), therefore, an LPF 11a is inserted in the modulator 11, which is, for example, a transversal filter of an FIR (finite impulse response) type as shown in FIG. 13. This LPF 11a extracts, for example, the baseband fc and a third harmonic 3fc located in regions indicated by dots in FIG. 10A.
Subsequently in a multiplier 11b in the modulator 11, when the carrier of 80 MHz is multiplied by the digital signal of 80 Mbps and modulated, as shown in FIG. 10B, the spectrum of a second harmonic 2fc is moved in frequency to the center carrier frequency fc of the baseband. From the modulator 11, a modulated signal 31 of 80 Mbpsxc3x972=160 Mbps as shown in FIG. 11B is thus output.
In order to effect pre-emphasis with an inverse characteristic to the transmission characteristic curve 32 of the coaxial cable 6 shown in FIG. 9B and thereby make the frequency characteristic on the receiving side (the side of the camera control unit 9) flat up to a high frequency component, a pre-emphasis characteristic 33 must be such that the characteristic in the transmitting end has a lower level (attenuation amount) on the side of the low frequency component f1 with respect to the center carrier frequency fc and a higher level on the side of the high frequency component f2 as shown in FIG. 9C.
Assuming that such a pre-emphasis characteristic 33 is provided, the spectra of FIG. 10B become as shown in FIG. 10C. For producing the spectrum having such a transmitting end characteristic which is asymmetric with respect to the center carrier frequency fc of the baseband, it is necessary to supply the modulated signal (FIG. 11B) of 160 Mbps output from the modulator 11 to a zero point insertion circuit 12 as shown in FIG. 8, conduct sampling at sampling points 35 located on zero points as shown in FIG. 11C, and conduct digital signal processing at at least 4fc. In this way, the modulated signal 31 of 160 Mbps is supplied to a pre-emphasis circuit 13 as a zero point inserted waveform 36 of 320 Mbps as shown in FIG. 11C. This pre-emphasis circuit 13 is also formed by a FIR transversal filter as shown in FIG. 13.
A preemphasis output 37 of 320 Mbps obtained by the preemphasis conducted in the preemphasis circuit 13 becomes as represented by a discrete time waveform in FIG. 11D.
The pre-emphasized output signal is subjected to digital-analog conversion in a digital-analog converter (DAC) 14, then passed through a bandpass filter (BPF) 15, amplified by an amplifier 16, and then supplied to the wire transmission line such as the coaxial cable 6 or the like as the transmitting output. As a result, the pre-emphasized output signal is extracted as shown in FIG. 10D. The modulated video signal pre-emphasized in the baseband is canceled in its high frequencies at the receiving end. It thus becomes possible to supply a video signal having a flat frequency characteristic over the range of f1 to f2.
The longer the coaxial cable 6 becomes, the higher the high frequency component of the video signal must be raised before transmission in the above described conventional configuration. Therefore, a high frequency component 38 as shown in FIG. 12A is differentiated and whisker-like components such as an overshoot 39 or undershoots 40 increase as shown in FIG. 12B. Because of such disorder of the waveform, it becomes necessary to make the dynamic range of the amplifier and the like on the transmitting side large.
This makes the design of the transmission output amplifier difficult. In addition, this increases the number of word length bits in the modulator 11 of the DDS system. In high speed transmission involving only a DAC having a limited number, the quantity of preemphasis cannot be made very large. This resulted in a problem that the length of the coaxial cable which can be compensated by the preemphasis cannot be extended.
In addition, the frequency band characteristic must have an attenuation characteristic which is asymmetric with respect to the center carrier frequency fc as shown in FIG. 9C. As described before with reference to FIG. 10C, therefore, the digital signal processing frequency of 4fc is required. This resulted in a problem that the power dissipation increased accordingly and the circuit scale had to be made large.
The present invention is to provide a video signal transmission apparatus and a video signal transmission method which solve the above described problems. An object of the present invention is to obtain a video signal transmission apparatus and a video signal transmission method which minimizes the increase of the dynamic range of a transmitting side output, which reduces the processing frequency required when forming the preemphasis circuit by using a FIR transversal filter to half as compared with the prior art, and which thus saves the power dissipation.
A video signal transmission apparatus according to the present invention includes an amplitude modulation means 11 for amplitude-modulating a carrier by using digital video data, a preemphasis means 13 for applying preemphasis to a modulated signal 31 modulated by the modulation means 11, a digital-to-analog conversion means 14 for converting output data pre-emphasized by the preemphasis means 13 to an analog video signal, and a wire transmission line 6 for transmitting the analog video signal supplied from the digital-to-analog conversion means 14 to a receiving end, as shown in its one example in FIG. 1, and the preemphasis means 13 conducts the preemphasis with a frequency characteristic which is the inverse of a frequency characteristic obtained when the analog video signal transmitted via the wire transmission line 6 without being pre-emphasized is detected at the receiving end.
According to the present invention, the zero point insertion circuit becomes unnecessary. The processing frequency required when forming the preemphasis circuit by using a digital filter can be reduced to half as compared to the prior art. Thus the power can be saved.